1. Field of the Invention
The invention relates to methods and apparatus for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improved engine and/or vehicle performance, such as improved vehicle fuel economy or reduced overall vehicle emissions.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (xcex), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of engine-generated constituent gases, such as HC, CO and NOx, that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Significantly, in order to maximize the NOx-storage capacity of the trap, it is important to fully purge the trap of stored NOx. The prior art teaches use of a xe2x80x9cswitchingxe2x80x9d oxygen sensor (HEGO) positioned downstream of the trap, by which to detect, during a purge event, a change of the downstream exhaust gas from a near-stoichiometric air-fuel ratio to a rich air-fuel ratio, at which point the trap is believed to be xe2x80x9cpurgedxe2x80x9d of stored NOx. Because excess fuel remains in the engine""s exhaust system, upstream of the trap, at the time at which the downstream HEGO sensor xe2x80x9cswitches,xe2x80x9d the trap receives an unnecessary, additional amount of rich exhaust gas, even if the engine operating condition is immediately returned to either stoichiometry or to lean operation. Accordingly, the prior art teaches the use of time-based correction of the purge time otherwise defined by the switching HEGO sensor, to thereby reduce the amount of remaining excess fuel upstream of the trap. Unfortunately, such time-based corrections fail to accommodate changes in intake space-velocities due to exhaust pressure, exhaust temperature and air mass flow, thereby limiting the effectiveness of such time-based corrections in addressing the fuel economy penalty and associated rich tailpipe exhaust characteristic of such HEGO-switching-timed systems.
It is an object of the invention to provide a method and apparatus for maximizing the fuel economy benefit to be obtained through lean-burn operation of an internal combustion engine by determining the amount of excess fuel remaining in the engine""s exhaust system, upstream of the trap, upon release of substantially all stored exhaust gas constituents from the trap.
In accordance with the invention, a method is provided for controlling the operation of an internal combustion engine in a motor vehicle, wherein the engine generates exhaust gas including at least a first and a second exhaust gas constituent, and wherein exhaust gas is directed through an emissions control device before being exhausted to the atmosphere, the device storing at least the first exhaust gas constituent when the exhaust gas directed through the device is lean of stoichiometry and releasing previously-stored first exhaust gas constituent when the exhaust gas directed through the device is rich of stoichiometry. The method includes, upon discontinuance of a rich engine operating condition of a predetermined duration, determining a value related at least in part to the presence of the second exhaust gas constituent in the exhaust gas downstream of the device; and calculating the difference, if any, by which the determined value exceeds a reference value, for example, a stoichiometric value. The method further includes accumulating the difference until the determined value is substantially equal to the reference value.
In an exemplary embodiment, the method further includes adjusting the duration of the rich engine operating condition based upon the accumulated difference, preferably by reducing the duration until the accumulated difference is less than a predetermined threshold value.
Under the invention, a controller is also provided for controlling an engine operating in combination with an emissions control device that releases a previously-stored first exhaust gas constituent when the engine is operated at a rich operating condition for a predetermined duration. The controller is arranged to determine a value related at least in part to the presence of a second exhaust gas constituent in the exhaust gas downstream of the device upon discontinuance of the rich operating condition. The controller is further arranged to calculate the difference, if any, by which the determined value exceeds a reference value, such as a reference value which approximates a stoichiometric value, and to accumulate the difference until the determined value is substantially equal to the reference value.
In the exemplary embodiment, the controller is further arranged to adjust the duration of the rich engine operating condition based upon the accumulated difference, preferably by reducing the duration until the accumulated difference is less than a predetermined threshold value.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.